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traps. He argues that our tendency to fall into social traps results from the speed of cultural evolution: societies change so rapidly that it is difficult to incorporate long-term considerations into day-to-day decisions. He argues that in order to eliminate social traps, short-term incentives must be brought into correspondence with long-term incentives.
Norton (in this volume) argues that decisions with long-term or widespread implications should be based on different criteria than decisions whose implications are only local or ephemeral. He contrasts the individual's roles as a consumer and as a member of a constitutional convention. Norton argues that while the consumer makes economic decisions based on individual utility in the relatively short run, the delegate to a constitutional convention must make decisions on the basis of the long-term best interests of the nation whose constitution is being drafted. In other words, different priorities must serve as the basis for decisions that have the potential for long-term effects, whether the issue is a nation's constitution or its environment.
This is not to suggest that the short-term problems discussed above should not have been "solved," merely that those searching for solutions should recognize the potential for "revenge" on the part of the system. As Garrett Hardin (1993:16) suggests, when contemplating the effect of a particular solution, one should ask, "And then what?" We believe that the following papers will help as engineers and ecologists consider Hardin's question in the course of future attempts to engineer within ecological constraints.
These remarks are based on discussions and presentations during an April 1994 National Academy of Engineering meeting on engineering within ecological constraints. George Diggs, Alexander Flax, Hugh MacIsaac, Deanna Richards and three anonymous reviewers provided valuable comments on earlier drafts. Credit for any insights should be attributed to the meeting participants.
Many definitions of sustainability have been proposed or implied. We use the term to refer to situations where the environmental impacts of present human activities do not reduce the potential for the environment to support future human activities. The laws of thermodynamics preclude truly infinite sustainability, but we consider this unimportant. With so many systems so far from sustainability at present, the concept is most useful as a guide in efforts to shift actions onto paths that appear to be more sustainable. Costanza (in this volume) elaborates on our conventional if loose definition by defining sustainability in terms of the expected life span of a system. Under his definition, if a system attained its expected life span, then it was sustainable. A species has a longer expected life span than a population, which has a longer expected life span than an individual, which has a longer expected life span than a cell.
Assimilative capacity is the rate at which the environment can render wastes innocuous.
New technologies could conceivably also have unanticipated desirable consequences.